The natural lens of a human eye is a transparent crystalline body, which is contained within a capsular bag located behind the iris and in front of the vitreous cavity in a region known as the posterior chamber. The capsular bag is attached on all sides by fibers, called zonules, to a muscular ciliary body. At its rear, the vitreous cavity, which is filled with a gel further includes the retina, on which light rays passing through the lens are focused. Contraction and relaxation of the ciliary body changes the shape of the bag and of the natural lens therein, thereby enabling the eye to focus light rays originating from objects at various distances on the retina.
Cataracts occur when the natural lens of the eye or of its surrounding transparent membrane becomes clouded and obstructs the passage of light resulting in varying degrees of vision impairment. To correct this condition in a patient, a surgical procedure is performed in which the clouded natural lens, or cataract, is extracted and replaced by an artificial intraocular lens. During cataract surgery, the anterior portion of the capsular bag is removed along with the cataract, and the posterior portion of the capsular bag, called the posterior capsule, is sometimes left intact to serve as a support site for implanting the intraocular lens (IOL). Such conventional IOLs, however, have the drawback that they have a fixed refractive power and are therefore unable to change their focus in response to changing focal distance needs of the patient, such as reading, or working on a computer.
Various types of intraocular lenses having the capability of altering their refractive power have been suggested in an effort to duplicate the performance of the natural lens within the eye. Such accommodating intraocular lenses, as they are known in the art, have a variety of designs directed to enable the patient to focus on, and thereby clearly see, objects located at a plurality of distances. Examples may be found in such publications as U.S. Pat. Nos. 4,254,509; 4,932,966; 6,299,641; 6,406,494, and 7,261,737.
U.S. Pat. No. 5,443,506 to Garabet discloses a variable focus intraocular lens which alters the medium between the two surfaces of the lens to alter its accommodation. The lens of the '506 patent has continuous flow loops which couple a channel in first portion of the intraocular lens. The continuous flow loops, in addition to providing a channel, provide the means by which the intraocular lens is positioned and held in the eye. In one embodiment, the continuous flow loop(s) comprise the lens haptics i.e., the lens body supporting structures, which, in turn move a charged solution into an optical zone between solid lens surfaces to change the lens focus.
U.S. Pat. No. 5,489,302 discloses an accommodating intraocular lens for implantation in the posterior chamber of the eye. This lens comprises a short tubular rigid frame and transparent and resilient membrane attached thereto at its bases. The frame and the membranes confine a sealed space filled with a gas. The frame includes flexible regions attached via haptics to the posterior capsule. Upon stretching of the capsule by the eye's ciliary muscles, the flexible regions are pulled apart, thereby increasing the volume and decreasing the pressure within the sealed space. This changes the curvature of the membranes and accordingly, the refractive power of the lens.
U.S. Pat. No. 6,117,171 discloses an accommodating intraocular lens which is contained inside an encapsulating rigid shell so as to make it substantially insensitive to changes in the intraocular environment. The lens is adapted to be implanted within the posterior capsule and comprises a flexible transparent membrane, which divides the interior of the intraocular lens into separate front and rear spaces, each filled with a fluid having a different refractive index. The periphery of the rear space is attached to haptics, which are in turn attached to the posterior capsule. Upon stretching of the capsule by the eye's ciliary muscles, the haptics and hence this periphery is twisted apart to increase the volume of rear space and changes the pressure difference between the spaces. As a result, the curvature of the membrane and accordingly, the refractive power of the lens, changes.
Another approach to varying the focus of an IOL is to form a conventional hard intraocular lens with a flexible outer surface made from a material such as silicone. Water is then injected in between the conventional hard portion of the lens and the flexible outer surface of the lens. The water will stretch the outer flexible layer to change the radius of curvature of the intraocular lens and thereby change the accommodation of the lens. One disadvantage of this approach is that a fluid source, a fluid pump and a flow control valve all must be provided within close proximity to the lens. As the area around the crystalline lens of the eye is quite confined, most of the fluid injection components have to be provided on the lens itself. Further, an energy source must be provided to pump the fluid. As there is no mechanical force generated in the eye that is strong enough to pump the fluid, an external power supply is required to run the pump. Such an external power supply is usually implemented using a battery which has a limited life cycle.
A further approach that has been used to vary the accommodation of an IOL is the coating of a conventional IOL with a liquid crystal material. A voltage source is applied to the crystal material to polarize the crystals. Once the crystals are polarized the refractive index of the crystalline material changes thereby changing the accommodation of the IOL. One principal disadvantage of this type of system is the relatively large amount of energy that is required to polarize the liquid crystal material, on the order of 25 volts. As there is no known manner of generating that level of voltage within the body, an external power source, such as a battery, is therefore necessary.
Some conventional accommodating IOLs rely on a solid curved surface to provide refraction. As such the force required for a change of curvature significant enough to induce an increase in diopter and accommodating power is much larger than that provided by the ciliary muscles especially in an aging lens. Other accommodating IOLs involve a displacement of the whole IOL along the optical axis to create accommodation. This does not only require a relatively larger force but also fails to deliver larger changes in diopter due to the lack of space in the anterior chamber.
The above described and other prior attempts to provide an intraocular lens with variable accommodation are generally complex systems. These complex systems are costly and difficult to manufacture and often times impractical to implement in the eye of a human. Therefore, current accommodating lenses provide little accommodating power (about 1 to 2.5 diopters “D”). For purposes of this art, it is to be understood that a multi-focal lens is not necessarily an accommodating lens, cf., U.S. Pat. No. 7,229,475. A true accommodating lens which more nearly mimics natural lens accommodation should have at least about 4 D, preferably at least about 6 D or more of accommodating power. In addition, multifocal IOLs provide a limited number of distances at which vision is adequate. In contrast, accommodating IOLs allow the patient to achieve good vision at any distance by providing a continuum of lens focal lengths to meet the wearer's needs. Therefore, a need exists for a simple IOL with greater levels of accommodating power that relies only on the forces provided by the human eye for operation.